(a) Field of the Invention
The present invention relates to liquid crystal displays using vertical alignment and compensation films.
(b) Description of the Related Art
A liquid crystal display has two substrates opposite each other and a liquid crystal layer interposed between the substrates. If the electric field is applied to the liquid crystal layer, the liquid crystal molecules changes their orientations to control the transmittance of the incident light.
A twisted nematic (TN) liquid crystal display includes a couple of transparent substrates having transparent electrodes thereon, a liquid crystal layer interposed between the substrates, and a couple of polarizers which are attached to the outer surfaces of the substrates. In off state, i.e., in absence of the electric field, the molecular axes of the liquid crystal molecules are aligned parallel to the substrates and twisted spirally by a constant pitch from one substrate to the other substrate, and the director of the liquid crystal layer varies continuously.
However, the contrast ratio of the conventional TN mode liquid crystal display, especially in normally black mode, may not be so high because the incident light is not fully blocked in absence of the electric field.
To solve this problem, a vertically aligned twisted nematic (VATN) mode liquid crystal display is proposed in the U.S. Pat. No. 5,477,358, xe2x80x9cCHIRAL NEMATIC LIQUID CRYSTAL DISPLAY WITH HOMEOTROPIC ALIGNMENT AND NEGATIVE DIELECTRIC ANISOTROPYxe2x80x9d, whose patentee is Case Western Reserve University and in xe2x80x9cEurodisplay ""93xe2x80x9d, pp. 158-159 by Takahashi et al.
On the contrary to the TN mode, the alignment of the liquid crystal molecules of the VATN mode liquid crystal display in off state is similar to that of the TN mode in on state, that is, the liquid crystal molecules align perpendicular to the substrates. In the on state, the molecular axes of the liquid crystal molecules are aligned parallel to the substrates and twisted spirally by a constant pitch from one substrate to the other substrate, and the director of the liquid crystal layer varies continuously.
In case of VATN mode liquid crystal display in normally black mode, sufficient darkness in off state because the molecular axes of the liquid crystal molecules are aligned vertically to the substrate when the electric field is applied.
The display characteristics of VATN may become better by optimizing the parameters such as the ratio d/p of the cell gap d to the pitch p of the liquid crystal layer, the difference of refractive indices xcex94n between in both directions and the retardation value xcex94n*d.
In the mean time, because of the refractive anisotropy of the liquid crystal material, the retardation value xcex94n*d changes as the viewing direction, thereby causing the difference of the intensity and the characteristics of light. Therefore, TN displays have the change in contrast ratio, color shift, gray inversion, etc. according to the viewing angle.
TN LCDs with compensation film are developed to compensate the difference of retardation in liquid crystal layer. However, the film compensated TN LCDs still have problems such as inharmony of the display characteristics and gray inversion.
It is therefore an object of the present invention to provide optimized cell parameters such as d/p, xcex94n and xcex94n*d to improve the optical characteristics fo LCDs.
It is another object of the present invention to widen the viewing angle of liquid crystal displays.
It is another object of the present invention to increase the contrast ratio of liquid crystal displays.
These and other objects, features and advantages are provided, according to the present invention, by a liquid crystal display comprising two parallel spaced substrates and a liquid crystal layer with negative dielectric anisotropy injected between two substrates wherein the ratio d/p, the cell gap d between two substrates to the pitch p of the liquid crystal layer, may be equal to or less than 0.3.
The liquid crystal layer is made of a chiral nematic liquid crystal or a nematic liquid crystal with 0.01-1.0 wt % of chiral dopant.
On two substrates, alignment layers are formed to align the liquid crystal molecules vertically to the substrates. The alignment layers may or may not be rubbed.
The refractive anisotropy xcex94n may be 0/065-0.123, the cell gap d between two substrates may be 3.0-6.0 xcexcm and the retardation value xcex94n*d may be 0.25-0.4.
When the electric field is not applied, the liquid crystal molecules are arranged vertically to the substrates, and when the sufficient electric field is applied, the liquid crystal molecules are parallel to the substrates and twisted by 90xc2x0 from one substrate to the other.
These and other objects, features and advantages are also provided, according to the present invention, by a liquid crystal display comprising a liquid crystal cell having a liquid crystal material with a negative dielectric anisotropy, and a combination of a-plate, c-plate or biaxial compensation films attached to the outer surface of the liquid crystal cell.
The slow axis which is the direction having the largest refractive index of a-plate or biaxial compensation film may be parallel or perpendicular to the transmission axis of adjacent polarizer.
The difference between the summation of the retardation (nxaxe2x88x92nza)*dB of the a-plate compensation film, the retardation (nxcxe2x88x92nzc)*dc of the c-plate compensation film, the retardation (nxbxe2x88x92nzb)*db of the biaxial compensation film and the retardation of the polarizers, and the retardation of the liquid crystal cell may be equal to or less than 15% of the retardation of the liquid crystal cell. The retardation (nxaxe2x88x92nya)*dB of the a-plate compensation film or the retardation (nxbxe2x88x92nyb)*db of the biaxial compensation film may be 0-100 nm. Here, nx, ny and nz are the refractive indices of the x, y and z axes respectively when z axis is the direction perpendicular to the surface of the liquid crystal cell, x axis is in the surface of the liquid crystal cell and having the largest refractive index of the a-plate or the biaxial compensation film and y axis is in the surface of the liquid crystal cell and perpendicular to the x axis, and d is the thickness of the liquid crystal cell.